1. Field of the Invention
The present invention relates to a novel method and apparatus for simultaneously and continuously analyzing a plurality of sample fluids using only one detector by a multi-fluid modulation mode (this is a name given by the present inventors).
2. Description of the Prior Art
Methods and apparatuses in which a fluid analyzer adopting various kinds of sensors is used, such as an analyzer provided with a chemical luminescence detector (CLD), an analyzer provided with a flame ion detector (FID), and a nondispersive type infrared analyzer (NDIR) provided with a pneumatic detector of condenser microphone sensor or microflow sensor or a solid detector such as a thermopile or semiconductor, have been known for analyzing a concentration (and thus a quantity) of noxious ingredients (such as NO.sub.x, H.sub.y C.sub.z or CO.sub.x) in an exhaust gas from cars and plants contained in an atmosphere as one example of a sample fluid.
However, where the fluid analysis is required to simultaneously and continuously measure concentrations of a plurality of ingredients (for example, two ingredients such as NO and NO.sub.2, methane (CH.sub.4) and HC other than methane (nonmethane hydrocarbon) (NMHC) or CO and CO.sub.2) contained in the sample fluid, a plurality of sensors (in this case, two sensors) have been required.
In the case where NO and NO.sub.2 are simultaneously and continuously measured, the sample fluid is divided into two measuring systems: a first NO sensor for measuring a concentration of NO contained in the sample gas being disposed in one system, and a second NO sensor for subjecting NO.sub.2 contained in the sample gas to a treatment for turning NO.sub.2 into NO and measuring a concentration of total NO formed in the treated fluid being disposed in the other system. That is, two NO sensors are required. The concentration of NO.sub.2 is obtained as a difference between a value of the concentration of total NO detected by the second NO sensor and a value of the concentration of only NO detected by the first NO sensor. This procedure is called a differential method. In the case where methane and HC other than methane (NMHC) are simultaneously and continuously measured, the sample fluid is divided into two measuring systems: a first HC sensor for measuring a concentration of total HC (THC) contained in the sample fluid being disposed in one system, and a second HC sensor for subjecting HC other than methane contained in the sample fluid to a treatment for catalytically burning and removing HC other than methane and measuring a concentration of methane contained in the treated gas being disposed in the other system. That is, two HC sensors are required. Also in this case, the differential method is used; that is, NMHC is obtained as a difference between a value of the concentration of THC detected by the first HC sensor and a value of the concentration of methane detected by the second HC sensor. In the case where CO and NO.sub.2 contained in the sample fluid are simultaneously and continuously measured, the sample fluid is divided into two measuring systems, a CO sensor being disposed in one system, and a CO.sub.2 sensor being disposed in the other system; that is, two sensors are required.
It is obvious that a plurality of sensors are required in the case where a plurality of ingredients contained in the sample fluid are simultaneously and continuously analyzed by dividing the sensors among a plurality of systems, as above described. It is also required in the case where a specific ingredient contained in a plurality of different sample fluids is simultaneously and continuously analyzed.
3. Problems to be Solved by the Invention
However, the requirement of a plurality of sensors as in the conventional methods and apparatuses for the simultaneous and continuous analysis of a plurality of ingredients contained in one fluid or the simultaneous and continuous analysis of the specific ingredient contained in a plurality of different sample fluids, as above described, leads to various kinds of problems. Among other things, (a) the analyzer is large in size and the cost of production is high; (b) since the calibration, such as zero-span calibration, is required for each of the sensors, much time is taken for the measurement; and (c) in the case where the calibration of the sensors is insufficient and there is an error in zero-calibration and a difference in sensitivity among the sensors, a large error of measurement is brought about.
In order to avoid such problems, a so-called batch type analyzing method has been used, in which a plurality of ingredients contained in one sample fluid are alternately determined or a plurality of different sample fluids are alternately measured by the use of an analyzer provided with only one sensor. But in such case, simultaneous and continuous measurement cannot be achieved, so that a disadvantage occurs in that the measured data is discontinuous. In particular, in the case where the analysis is carried out by the use of the differential method, there is the possibility that the accuracy of measurement is greatly reduced. Accordingly, it leads to such a great sacrifice of analysis of a fluid that a batch type analyzing method is used for merely reducing a number of sensors, which is not the best approach.
The present invention was achieved in view of such matters, and thus it is an object of the present invention to provide a method and apparatus for simultaneously and continuously analyzing a plurality of sample fluids with high accuracy by the use of only one sensor.